In particular,
\(\iota (T_1)\subset \mathcal {A}_{2,1} \subset \mathcal {A}_3\) , with
\(\mathcal {A}_{2,1}\)  the moduli of abelian varieties that are
products.
The equations \(\tau _{11}=\tau _{22}\)  and \(\tau _{11}+2\tau _{12}=0\) 
define two Humbert surfaces of discriminant 4 in \(\mathcal {A}_2\) ,
cf. [1]}.
